Turbocharged engines utilize a turbocharger to compress intake air and increase the power output of the engine. A turbocharger may use an exhaust-driven turbine to drive a compressor which compresses intake air. As the speed of the compressor increases, increased boost is provided to the engine. During transient conditions, upon receiving an increased torque demand, there may be a delay in turbocharger response before the turbine and compressor speed is increased to a desired speed necessary to provide the required boost. This delay in turbocharger response, termed turbo lag, may result in a delay in providing the demanded engine power. For example, during vehicle launch conditions, such as when accelerating from idle, minimal exhaust gas flow combined with increased load on the compressor may result in turbo lag. Consequently, when accelerating from idle speed, turbo lag may decrease responsiveness of the vehicle to driver's torque demand, and thus decrease driving control.
One example approach for reducing turbo lag is shown by Pallett et al. in U.S. Pat. No. 8,355,858 B2. Herein, in addition to a first fuel injection, a second fuel injection is performed after combustion, during the same cylinder cycle. The un-combusted fuel from the second fuel injection is delivered into the exhaust upstream of the turbine, thereby providing increased heat to increase turbine speed.
However, inventors herein have identified issues with such an approach. For example, providing un-combusted liquid fuel in the exhaust produces increased soot and particular matter. Additionally, exhaust heat may be lost due to heat transfer in the combustion chamber. As a result, performing the second fuel injection as described by Pallett may result in degraded fuel economy and emissions.
In one example, the above issues may be addressed by an engine method comprising: combusting a first amount of gaseous fuel during a compression stroke of a cylinder combustion event using a first ignition energy; and combusting a second amount of gaseous fuel during an exhaust stroke of the cylinder combustion event using a second ignition energy, the second ignition energy lower than the first ignition energy.
As an example, an engine system may be configured with a liquefied petroleum gas (LPG) fuel delivery system and the gaseous fuel (e.g., LPG) may be direct injected into the combustion chamber. Based on engine operating conditions, such as if a torque demand increase is greater than a threshold, a second fuel injection with spark ignition may be performed to reduce the time required to increase turbine speed to a desired speed. Specifically, a first lean intake stroke injection may be performed, followed by spark ignition during a compression stroke of a cylinder combustion event. Subsequently, during the same cylinder combustion event, a second fuel injection may be performed and combusted by spark ignition during an exhaust stroke of the cylinder combustion event. An amount of the second fuel injection may be adjusted to maintain an overall air-fuel ratio at stoichiometry or slightly rich. Further, an ignition energy of the spark provided for combustion may be adjusted for efficient and complete combustion of the second fuel injection, thereby reducing parasitic losses. As such, the ignition energy of the ignition spark provided for combustion of the second fuel injection may be lower than the ignition energy of the ignition spark provided for combustion of the first fuel injection. The ignition energy may be adjusted by adjusting one or more of an ignition coil dwell time and/or an ignition strike frequency.
In this way, additional exhaust energy may be produced by spark igniting the second fuel injection. The additional exhaust energy may then be utilized to increase turbine speed to a desired speed. Upon achieving a desired turbine speed and/or manifold absolute pressure (MAP), the engine may be operated without the post fuel injection.
Additionally, injecting and igniting fuel during the exhaust stroke of a cylinder combustion event may reduce the duration to accelerate the turbocharger to a desired speed and provide the desired boost. As a result, turbo lag may be reduced while also reducing a loss of heat to the combustion chamber and particulate matter formation. Further, by adjusting ignition energy for combustion of the second fuel injection amount, combustion may be controlled and parasitic electrical energy losses may be reduced.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.